In a harbour or navigable channel the mud layer generally consists of a fluid mud layer and a consolidated (solid) mud layer. The transition between these layers is a rheological transition level which can be deemed as the real nautical bottom. It is particularly important for shipping traffic to know the exact depth at which this transition is located.
The prior art already comprises a number of techniques for measuring the nautical bottom, wherein a distinction is made between a measurement in which an object is towed, and a sounding/penetration measurement at a determined location.
The Belgian patent with publication number 1015773 in the name of applicant thus describes a solution in which the nautical bottom is measured using a continuously towed random body.
It is also known to use acoustic signals to determine the depth of the rheological transition level. As illustrated in FIGS. 1A and 1B, this leads to inaccurate results. At a relatively high frequency of 210 kHz the acoustic signal is reflected at the top of the fluid mud layer 8 and not at the rheological transition level 10. At lower frequencies the level at which reflection takes place is uncertain. Such a measurement does not therefore allow of accurate determination of the location of the transition.
Further known is a so-called Navitracker which performs an up and downward movement in the mud. The density of the mud layer is measured in the Navitracker using a radioactive source. The Navitracker cannot however be introduced into the solid mud layer and cannot therefore determine the rheological transition level. An analogous technique is used in the so-called sounding/penetration probes, where the probe is carried through the fluid mud layer to a position in the vicinity of the solid mud layer while the density is measured. The density at the end of the fluid mud layer 8 is however substantially the same as the density in the solid mud layer 9 (see FIG. 1A), so that the rheological transition cannot be accurately determined.